1. Field of the Invention
This invention relates to insulated pipelines, pipeline insulating materials and particularly to a more cost effective, abrasion resistant, insulated offshore pipeline.
2. Related Art
At low temperatures, the flow through pipelines can be impeded by high viscosity and wax formation in liquid products such as oil, and by hydrate formation in products such as natural gas. These problems can be reduced by using thermally insulated pipelines, but insulated pipelines are expensive on land and even more costly offshore. For offshore pipelines it has usually been more cost effective to reduce the need for insulation by injecting various chemicals into the product. Recently, however, more and more oil and gas is being produced in deeper, colder water, from sub-sea production systems where use of viscosity reducing chemicals requires a dedicated line to transport them to the wellhead. This, combined with the fact that the cost of insulating pipelines typically increases with depth, means that insulated pipelines are most expensive where the alternatives are least attractive.
Various materials have been used to insulate land pipelines, including expanded cork, polymer foams, calcium silicate and others. Insulating pipelines offshore is somewhat more complicated because most insulating materials derive their low conductivity from voids which can become saturated in water. Some insulating materials incorporate watertight closed cell structures, but all have some depth limit at which the cellular structure will collapse, and most will fail in a few hundred feet of water. Furthermore, most common insulating materials have little resistance to impact, abrasion or crushing, and must therefore be encased. If the water depth exceeds the hydrostatic pressure limitations of the material then the casing must also isolate the insulating material from the hydrostatic head of the water.
If the pipeline is laid in sections it is a practical necessity to prefabricate each individual pipe section as an independent pressure vessel. Because pressure resistant double pipes are too stiff to spool, several reel laid pipelines have been installed with flexible coatings of solid, elastomers or elastomers filled and extended with other lightweight materials. Examples include neoprene and EPDM rubber, EPDM and polyurethane elastomers filled with glass micro-spheres, and ebonite filled with cork. Unless the insulation requirement is minimal, the total cost of pipelines insulated in this manner is even higher than one which uses a pressure resistant casing to protect less expensive insulating materials. These elastomers cost three to four times as much as thermoplastics such as polypropylene, but the demand is such that extrusion equipment that would be needed to apply thermoplastic materials would have to be amortized over relatively few jobs. This might well outweigh the cost benefit of the material. Another disadvantage of plastics and elastomers is that the operating temperature limits of many are in the range of 190.degree. to 250.degree. F.
The problem of protecting pipeline corrosion coatings against impact and abrasion arises anywhere the pipeline is laid in or towed across rocky terrain. One way to protect corrosion coatings is with a layer of concrete, but this has limited application due to the added weight. On offshore pipelines, concrete is commonly applied to the pipeline to add weight so that it will not float when it is empty. There are two types of equipment in common use for applying concrete to line pipe. One employs what is known as the "impingement method", whereby the wet concrete is sprayed or slung with rotating brushes so that it hits a rotating and axially moving pipe. The other method involves depositing concrete on a wide tape which is being spirally wrapped while rollers outside the tape extrude the concrete to a uniform thickness.
Normal weight concrete is typically not a very good insulator even when dry and worse when saturated. Lightweight concrete mixes are often used for insulation in many applications. Some of these mixes depend on various air entraining chemicals while others incorporate lightweight cellular aggregates. Some of these are closed cell aggregates such as expanded pearlite, but even these closed cell aggregates generally absorb water, and the porosity of the cement binder is considered to be an advantage because it reduces the conductivity. All of these insulating concretes are much more porous than standard weight concrete, and the conductivity goes up exponentially with water absorption. Published data on an expanded shale lightweight concrete with a dry density of approximately 98 lb per cubic foot shows that the conductivity of a saturated sample is nearly three times as high as the same sample after it was oven dried. Furthermore, the amount of material needed to maintain a given temperature drop on an insulated pipeline increases exponentially with the thermal conductivity of the material. Finally, the high hydrostatic pressures in deep water maximize the water absorption a given amount of porosity will allow. Coating these materials does not easily solve this because of the inherent lack of flexibility of most lightweight concrete mixtures. To keep the water out, the coating would have to be able to bridge the cracks that develop while the pipeline is being laid.
U.S. Pat. No. 3,782,985 discloses that hollow, lightweight components of fly ash known as cenospheres can be mixed with hydraulic cements to produce materials with low density, low porosity, low conductivity and high strength. Similar results can be achieved using glass microspheres and certain other closed cell aggregates with high compressive strength.
It is known in the prior art that fumed silica can reduce the porosity and increase strength of concrete. U.S. Pat. No. 4,505,320 discloses that the addition of substantially equal amounts of fumed silica and cenospheres with glass fibers to a hydraulic cement increases compressive and tensile strength. The silica fume does substantially reduce water absorption and the fibers increase tensile strength, but this material would not be normally be suitable as a general purpose insulating material because:
1. Silica fume increases the cost and density. Increasing density leads to a corresponding increase in conductivity under normal conditions. PA1 2. Insulating concretes are normally applied as laminates over more flexible materials, such as steel and are subject to flexing as the substratum expands or bends under load. Silica fume increase tensile and flexural strength but it decreases flexibility, and therefore leads to cracking of the insulation when the substratum bends. PA1 3. Insulating concretes use a higher ratio of porous aggregate to the total mix in order to achieve low conductivity. PA1 (a) 40 to 55 weight percent of a binder component having a total of 4 to 5 parts by weight comprising; PA1 (b) 40 to 22 weight percent fly ash cenospheres; PA1 (c) 1 to 2 weight percent super plasticizer; and PA1 (d) water
Both of the above mentioned patents reveal that the low permeability is achieved despite the fact that the required ratio of water to cement is higher than for normal concrete. Even with the high water content suggested in these patents, the flowability of the mix is poor. In fact, one reason, if not the primary reason for the high water requirement relates to the flowability of the paste, rather than any reactive requirement. This can be observed by the collection of expelled water on the surface as the material cures. The lack of flowability results from the fact that the weight is too low to overcome its viscosity. If the water content is increased to achieve better flowability, homogeneity of the mix is lost as the light weight cenospheres float to the top. Increasing water content also increases shrinkage that results in cracking when the material is cured on a pipe.
Among other things, the present composition differs from those described in U.S. Pat. Nos. 3,782,985 and 4,505,320, with the addition of polymeric modifiers or substitution of polymeric modifier for some or all of the silica fume to the material described in U.S. Pat. No. 4,505,320.
When fumed silica is added in high concentrations of 15 to 35 percent of the weight of cement, the porosity of the solidified mass becomes very low, but fine discontinuous "micro-cracking" occurs throughout the solidified material. That these micro-cracks can become the predominate factor in water absorption can be clearly observed with the naked eye after the cured material has been submerged for a short time. Cracks that were hardly visible when dry become clearly visible while the water beads on the surface. It has been suggested that micro-cracking may result from the fact that the porosity of the cement matrix is so low that cracks form during the expulsion of excess water. Whether or not this hypothesis fully explains the phenomenon, it is supported by the fact that the cracks are smaller and fewer when less water is used in the mix. Inclusion of fibers reduce microcracking.